Charged particle accelerator having a pressure range of 10**-5 to 10**-7 torr



Sept. 10, 1968 F. K. PETERSILKA ET AL 3,401,345

CHARGED PARTICLE ACCELERATOR HAVING A PRESSURE RANGE OF 1O-5TO 1O-7TORR Filed July 22, 1965 United States Patent Filed July 22, 1965, Ser. No. 474,022 Claims priority, applicti9o2n Ggrmany, July 24, 1964,

2 Claims. 01. 328-237) ABSTRACT OF THE DISCLOSURE A charged particle accelerator of the betatron type in which the radiation output is improved by adjusting and maintaining the vacuum in the tube to a value within the range of approximately 10- to l' torr, such that the amplitude of oscillations of the accelerated particles are maintained at a maximum without striking the tube walls.

The invention deals with a machine for the acceleration of charged particles in an evacuated acceleration tube, in particular with the betatron.

It is common knowledge that charged particles can be accelerated in such machines only if in the tube in which the particles are to be accelerated a pressure lower than a certain definite threshold, that is below 10- torr 1 approximately, exists. If the pressure is kept below this threshold, the radiation output obtainable is improved and this led to the conclusion that the improvement continues uniformly as the pressure decreases (see, for instance, Scattering Losses in the Synchrotron, by I. Greenberg and T. Berliwin Rev. Sc. Inst. 22/5, May 1951).

Contrary to this, the present invention is based on the surprising finding consolidated by numerous tests that the radiation output of an accelerator does not rise steadily as the pressure present in the acceleration tube decreases as was expected, but that it reaches a marked maximum value within the range from and 10* torr as a function of the pressure. The range in which maximum radiation output is attained depends on the oscillations of the particles during acceleration in a direction radial and axial to the acceleration path. These oscillations, however, are also determined by the injection and trapping process of the particles and by the sructure of the magnetic acceleration and magnetic guide field. For this reason the pressure range in which maximum radiation output occurs may slightly vary from one accelerator design to another.

In accordance with the invention, a charged particle accelerator, in particular a betatron in an evacuated acceleration tube, is characterized by the fact that in addition to means possibly provided to prevent the vacuum from deteriorating such other adequately dimensioned means are used that allow the pressure in the acceleration tube to be set to a value within the range of about 1() to 10- torr at which the accelerated particle flux reaches its maximum even with unavoidable leakages and gas sources present.

The above mentioned dependence of the oscillations utilized by the invention and effected by the particles in the acceleration tube oifers the following free possibility of explaining this effect. The collisions of the particles to be accelerated with the molecules of residual gas still present at a pressure of 10* to 10- torr attenuate the oscillations. In this manner the particle flux is kept together and a high radiation output is obtained. The eifect of keeping the particle flux together is brought about by the positive space charge generated by the ionization of the 1 1 t0rr:1 mm. Hg pressure.

3,401,345 Patented Sept. 10, 1968 "ice gas particles present. This space charge is concentrated in the middle of the tube so that the force keeping the trapped electrons together is maintained. However, if the pressure and consequently the number of residual gas molecules are reduced, the ionization and associated attenuation of the oscillations decrease as well. But when the attenuation diminishes, the oscillations increase and particles are likely to strike against the tube wall and to be lost to the acceleration. This occurs the more the lower the attenuation of the oscillations, i.e. the lower the pressure in the tube is. Also with higher pressures a lower radiation output is achieved because the particle flux is prevailingly decelerated by the gas molecules.

This supposition has been confirmed experimentally by measurements carried out on an electron accelerator. The measuring results are plotted on the graph shown in FIG. 1. This graph shows the dose rate as a function of the pressure prevailing in the accelerator tube. The pressure data are given on a logarithmical scale. The dose rate is indicated linearly in percentages of that dose rate which one would obtain if all particles injected into the acceleration tube would contribute to the dose rate obtainable. From the curve thus obtained it can be seen that the dose rate starts rising steeply from the 18% value reached at about 3 10- torr. The dose rate reaches the peak value of 98.5% at 6 1O- torr approximately. At 10- torr the dose rate has again fallen to 89% and continues dropping abruptly to reach 53% at 10" torr. This confirms the efiect, on which the invention is based, also experimentally.

FIG. 2 exemplifies a section of an accelerator designed in accordance with the invention, i.e. an electron accelerator. To the acceleration tube 1 a pressure gauge 2 and a pump 3 are connected. Pressure gauge 2 is coupled with the control unit 5 by the electric connection 4. The control unit 5 can be set to limit values and is connected with pump 3 via the electric leads 6. By means of the limited values set and indicated by pressure gauge 2 the pump 3, which is linked with the interior space of tube 1 through vacuum duct 7, can be switched on and off. As soon as the pressure in tube 1 has dropped sufliciently so that the pressure gauge 2 linked with tube 1 by the vacuum duct 8, indicates a magnitude corresponding to the lOWer limit, the pump 3 again is switched off. In this manner the desired pressure in the tube 1 is constantly maintained. Ordinarily the leakage present in the known tube 1 and providing for a constant deterioration of the vacuun is sufficient for the pressure setting.

The increase in pressure, however, can also be obtained by gas sources capable of being remote-controlled inside a leakproof tube. As suitable sources, especially material which emits gas when heated, can be used, such as metal parts which have not been degassed. A simple design is marked with 9 and shown in the drawing as a dotted section. It represents a filament which has not been degassed. When a voltage is switched on via lead 10 the heating of filament 9 can be efiected and regulated via control unit 5, that is in accordance with the pressure measurement made by pressure gauge 2. It is a known fact that non degassed metal parts emit gases when heated. As gas stored in the filament a readily ionizable gas, such as hydrogen or helium is preferentially used.

For the pressure reduction a known controllable getter can be used instead of the known pump. The getter can be one of zirconium which absorbs gas on heating. But the getter could also be of another material such as graphite which absorbs gas on cooling.

With an accelerator designed in accordance with the present invention it may prove advantageous to couple several getters or pumps, possibly of different suction speed. In this way it is possible to adjust the pressure reduction sensitively.

Moreover, in the tube 1 there are the electrodes 11 and 12 fitted to the interior edge and outer edge of the inner space of the tube. The electrode 11 is linked with the negative pole of power source 13 via lead 14 and represents therefore the cathode. The electrode 12 is connected via lead 15 to the positive pole of source 13 and is designed to dispose of the negative charge produced by the electron flux issuing from 11 so that the positive charges remain in the tube. This process of generating a positive space charge in the case of which the potential of the DC source 13 amounts to 5 up to 15 kv., and in the present case 11 kv., enables the positive space charge to be generated prior to injection and independent of the acceleration.

We claim:

1. An accelerator designed to accelerate charge particles in an evacuated acceleration tube of the betatron type in which the particles are caused to be accelerated along an orbit characterized by the fact that, electrodes within the tube are connected to a source of potential to generate a positive space charge by the ionization of the gas particles present, means to maintain the pressure in the acceleration tube at a value within a range of approximately 10* to 10' torr, whereby the amplitude of the oscillations of the electrons are maintained at a maximum amplitude without striking the tube walls.

2. An accelerator for charged particles in accordance with claim 1 characterized by the fact that the electrodes include an anode and a cathode which are disposed within the tube at opposite sides of the acceleration orbit therein adjacent opposite portions of the wall defining the tube.

References Cited UNITED STATES PATENTS 2,964,634 12/1960 Harris 31363 X 2,994,775 9/ 1961 Mott 313-63 X 2,880,373 3/1959 Soloway 313174 X 2,964,665 12/ 1960 Brinkerhoff et al.

FOREIGN PATENTS 480,410 1/1952 Canada.

DAVID I GALVIN, Primary Examiner. 

